Welding SM490 requires special attention to preserve its toughness and prevent cracking in the welded joint, especially since it is used in critical, dynamically loaded structures. The requirements are more stringent than for ordinary carbon steels like SS400/SS490.
Here are the key special requirements for the welding process of SM490:
1. Pre-Weld: Critical Preparation & Qualification
Welding Procedure Specification (WPS) Qualification:
A detailed WPS must be qualified before production welding, typically following JIS Z 3841 or international standards like ISO 15614-1.
Qualification must include Charpy V-notch impact testing of the weld metal and Heat-Affected Zone (HAZ) at the specified design temperature (e.g., 0°C for SM490B/C).
Material Certification & Traceability:
Verify the mill certificate to confirm the grade (A, B, or C) and actual chemical composition, particularly Carbon Equivalent (Ceq).
Joint Design & Preparation:
Use joint designs that minimize stress concentration (e.g., smooth transitions, avoid sharp notches).
Strict cleanliness: Remove all rust, moisture, oil, paint, and mill scale from the joint area (typically 25mm on each side). Contaminants introduce hydrogen, leading to cracking.
2. -Destructive (Most Important Step)
Low-Hydrogen Electrodes/Rods are MANDATORY.
Use JIS Z 3211 or AWS A5.1/A5.5 classified electrodes with an "H" designation (e.g., E7016, E7018, E10018-G).
Examples: For SMAW (stick): E7016, E7018. For FCAW/GMAW: ER70S-6 wire with matching low-hydrogen flux/gas.
Matching or Overmatching Strength:
The weld metal's yield/tensile strength should match or slightly exceed that of SM490 (≥ 490 MPa tensile). Typically, consumables for 490 MPa or 500 MPa class steels are used.
Toughness Requirement:
The consumable must guarantee impact toughness at a temperature equal to or lower than the base metal's requirement. For SM490C, select consumables certified for impact at 0°C or below.
3. Controlled Welding Parameters & Technique
Preheat & Interpass Temperature Control:
Preheat is often required, especially for thicker sections (typically >25-30mm), in cold ambient conditions, or for high restraint joints.
Purpose: Slows the cooling rate, allowing hydrogen to diffuse out and preventing the formation of hard, brittle martensite in the HAZ.
Typical Range: 50°C to 150°C, determined by the Ceq value, thickness, and restraint. SM490C, with its lower Ceq, may require lower preheat than SM490B for the same thickness.
Interpass Temperature must be controlled to a specified maximum (often 250°C) to prevent overheating and grain coarsening, which reduces toughness.
Heat Input Control:
Must be maintained within a qualified range. Excessive heat input can create a large, coarse-grained HAZ with low toughness. Very low heat input can cause excessive hardening.
The WPS will specify voltage, current, and travel speed limits.
Welding Technique:
Use stringer beads or slight weaving to manage heat input.
Ensure proper bead sequencing to minimize distortion and residual stress.
Crater filling must be complete to avoid crack initiation points.
4. Post-Weld Requirements
Post-Weld Heat Treatment (PWHT):
Not routinely required for all structures.
Mandatory for:
Very thick sections (as per code, often > 38-50mm).
Highly restrained joints.
Applications under severe fatigue or corrosive environments (e.g., offshore, pressure vessels).
Purpose: PWHT (e.g., stress relief at 550-650°C) reduces residual stresses, tempers the HAZ, and helps diffuse any remaining hydrogen.
Grinding of Weld Toes:
For fatigue-critical details (e.g., bridge connections), grinding weld toes to a smooth profile is required to remove stress concentrations and improve fatigue life.
5. Inspection & Testing (Verification)
Non-Destructive Testing (NDT):
100% visual inspection (VT) is standard.
Radiographic Testing (RT) or Ultrasonic Testing (UT) is mandatory for full-penetration butt welds in critical structures (e.g., bridges, seismic frames). The acceptance criteria are stricter than for non-structural work.
Destructive Testing for Qualification:
As mentioned, procedure qualification tests must include tensile, bend, and impact tests to verify joint integrity and toughness.
Special Considerations by Grade
SM490A: Has the least stringent requirements due to its lack of toughness guarantee. However, good low-hydrogen practice is still advised for thick sections.
SM490B & C: All the above special requirements apply fully. For SM490C, due to its use in the most critical applications, adherence is non-negotiable, and parameters (like heat input) are more tightly controlled.
Summary of "Special" vs. Ordinary Welding
| Requirement | For Ordinary Carbon Steel (e.g., SS400) | For SM490 (B/C) in Critical Structures |
|---|---|---|
| Consumables | Regular electrodes may be acceptable. | Low-Hydrogen (H-rated) mandatory. |
| Preheat | Often waived for thin sections. | Routinely required and controlled. |
| Heat Input Control | May not be specified. | Strictly controlled within a qualified range. |
| Procedure Qualification | May only test strength/ductility. | Must include impact toughness testing. |
| NDT Extent | Spot checks often suffice. | Extensive, often 100% on critical welds. |
In essence, welding SM490 is a highly controlled process designed to ensure the welded joint possesses the same level of toughness and integrity as the base metal, thereby maintaining the structural performance required for seismic, bridge, and other demanding applications.